1. Field of the Invention
This invention relates to control systems for helical screw rotary compressors.
2. Prior Art
This invention is an improvement over the pneumatic and hydraulic control system which is disclosed in Application Ser. No. 531,121, now U.S. Pat. No. 4,076,461 entitled "Feedback Control System For Helical Screw Rotary Compressors." In general, such screw compressors utilize an oil-flooded rotary screw compressor assembly which is directly coupled to an electric motor for rotation of the screw elements. Compression is achieved in the compressor section by meshing of two precision rotors, rotating in opposite directions inside a compressor chamber. In such screw compressors, a suction stroke occurs as a male lobe of one rotor leaves a female pocket in another rotor during its exposure to the port inlet area. The suction continues during rotation until a cut-off at the inlet port. This volume of air is then trapped and compressed as the male lobe meshes with the female pocket, thereby continuously reducing the trapped air volume and creating a pressure increase. Continued rotation exposes the internally compressed air to a discharge port which is then forced out of the machine as the male lobe completes its final meshing with the mating pocket of the female rotor. By varying the effective length of the compressor rotor, output pressure can be varied.
Prior art control of the effective length of the compressor was achieved by means of a hydraulic cylinder and piston assembly which was coupled by hydraulic piston to a sliding valve element. By selectively driving the hydraulic piston, the valve assembly was moved to control the effective length of the male and female rotors under compression in the screw compressor, thereby controlling compressor output. Control of the hydraulic piston was by means of a pneumatically operated sequencing valve. The pneumatic valve was used to divert oil to either the inboard or outboard side of the hydraulic piston and thereby effectively shift the control valve.
One of the difficulties with this prior art arrangement was a relatively wide control range (typically, 10 psi). Conventional suction throttled equipment also requires large pressure rises to complete unloading, typically 10 psi, and such a pressure increase when added to the pressure drop associated with after-cooling, separating drying plant piping and the like, can cause a pressure increase in the range of 10-18 psi before conventional equipment is completely unloaded. In contrast, by use of electronic control over a four-way solenoid valve, power consumption is minimized because the control can maintain air header pressure constant regardless of demand. Electronic control over the system will allow the compressor discharge pressure to fall as it unloads, while header pressure can remain constant. Because the compressor is used to hold system pressure at an essentially constant level independent of compressor output, the compressor discharge pressure can actually fall at a reduced flow, thereby avoiding the 10-18 psi pressure rise which conventional pneumatic controls require to merely minimize compressor output.
Another problem with conventional equipment is that electric motors used to drive the compressor section are built in size to minimum standards. Hence, minimum size motors are conventionally used which will require larger current demand into the service factor for normal operation. This minimum sizing, when coupled with contemporary voltage cutbacks and "brown-outs", tends to shorten motor life, and in extreme cases cause burn-out. By use of solid state circuitry, a load limiter can be used to prevent the motor from drawing more power than its assigned maximum service factor rating. When the current draw exceeds a set value, the compressor will unload until it reaches a point where current draw is equal to motor service factor rating. By use of load limiters, the system can be field adjusted such that compressor output would decrease but the drive motor will not draw more current than a predetermined amount, irrespective of how high of an increase in discharge pressure is set into the system. By this technique, the use of larger motors, starters and the like is eliminated because the load limiter functions as a real time mechanism to match motor current draw with system output.
Another problem in the prior art pneumatic control technique was the fact that the pressure tap which is used to provide a sensor input to the pneumatic sequencing valve had to be located at a position near the compressor element itself. Accordingly, the sensor could not be located at a point in the system where a user wished to maintain a constant minimum pressure. By the use of the novel control electronics in the present application, a pressure sensing element can be located in the air header at a point where a constant minimum pressure is to be maintained anywhere in the installed location. By locating the pressure-sensitive element at this point, the package discharge pressure can be reduced rather than increased with decreasing load. In conventional suction throttle equipment, the control sensing line is located at a point immediately downstream of the oil separator, and as a consequence, header pressure and compressor discharge must increase to a substantial amount to unload the compressor as the demand increases. As a consequence, an excess of header pressure results in wasting compressor drive energy in the system.